Mammal classification has been through several iterations since Carl Linnaeus initially defined the class. No classification system is universally accepted; McKenna & Bell (1997) and Wilson & Reader (2005) provide useful recent compendiums.[1]George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v. 85, 1945) provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics. Though field work gradually made Simpson's classification outdated, it remains the closest thing to an official classification of mammals.[2]

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latinmamma ("teat, pap"). In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes (echidnas and platypuses) and therian mammals (marsupials and placentals) and all descendants of that ancestor.[3] Since this ancestor lived in the Jurassic period, Rowe's definition excludes all animals from the earlier Triassic, despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century.[4] If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to the therians; as fossils of all three genera are dated about 167 million years ago in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group.[5]

T. S. Kemp has provided a more traditional definition: "synapsids that possess a dentary–squamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view, the clade originating with the last common ancestor of Sinoconodon and living mammals.[6] The earliest known synapsid satisfying Kemp's definitions is Tikitherium, dated 225 Ma, so the appearance of mammals in this broader sense can be given this Late Triassic date.[7][8]

In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the McKenna/Bell classification. Their 1997 book, Classification of Mammals above the Species Level,[9] is a comprehensive work on the systematics, relationships and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though molecular genetic data challenge several of the higher level groupings. The authors worked together as paleontologists at the American Museum of Natural History, New York. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.[2]

Estimates for the divergence times between these three placental groups range from 105 to 120 million years ago, depending on type of DNA (such as nuclear or mitochondrial)[13] and varying interpretations of paleogeographic data.[12]

Synapsida, a clade that contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod, when they split from reptilian and avian lineages. Crown group mammals evolved from earlier mammaliaforms during the Early Jurassic. The cladogram takes Mammalia to be the crown group.[18]

The original synapsid skull structure contains one temporal opening behind the orbitals, in a fairly low position on the skull (lower right in this image). This opening might have assisted in containing the jaw muscles of these organisms which could have increased their biting strength.

The first fully terrestrial vertebrates were amniotes. Like their amphibious tetrapod predecessors, they had lungs and limbs. Amniotic eggs, however, have internal membranes that allow the developing embryo to breathe but keep water in. Hence, amniotes can lay eggs on dry land, while amphibians generally need to lay their eggs in water.

Therapsids descended from pelycosaurs in the Middle Permian, about 265 million years ago, and became the dominant land vertebrates.[23] They differ from basal eupelycosaurs in several features of the skull and jaws, including: larger skulls and incisors which are equal in size in therapsids, but not for eupelycosaurs.[23] The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very similar to their pelycosaur ancestors and ending with probainognathiancynodonts, some of which could easily be mistaken for mammals. Those stages were characterized by:[25]

Progression towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was slow and erratic: for example, all herbivorous nonmammaliaform therapsids retained sprawling limbs (some late forms may have had semierect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semisprawling hindlimbs. In fact, modern monotremes still have semisprawling limbs.

The dentary gradually became the main bone of the lower jaw which, by the Triassic, progressed towards the fully mammalian jaw (the lower consisting only of the dentary) and middle ear (which is constructed by the bones that were previously used to construct the jaws of reptiles).

The Permian–Triassic extinction event, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of carnivorous therapsids.[26] In the early Triassic, most medium to large land carnivore niches were taken over by archosaurs[27] which, over an extended period (35 million years), came to include the crocodylomorphs,[28] the pterosaurs and the dinosaurs;[29] however, large cynodonts like Trucidocynodon and traversodontids still occupied large sized carnivorous and herbivorous niches respectively. By the Jurassic, the dinosaurs had come to dominate the large terrestrial herbivore niches as well.[30]

The oldest known fossil among the Eutheria ("true beasts") is the small shrewlike Juramaia sinensis, or "Jurassic mother from China", dated to 160 million years ago in the late Jurassic.[36] A later eutherian, Eomaia, dated to 125 million years ago in the early Cretaceous, possessed some features in common with the marsupials but not with the placentals, evidence that these features were present in the last common ancestor of the two groups but were later lost in the placental lineage.[37] In particular, the epipubic bones extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, nontherian mammals and Ukhaatherium, an early Cretaceous animal in the eutherian order Asioryctitheria. This also applies to the multituberculates.[38] They are apparently an ancestral feature, which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles during locomotion, reducing the amount of space being presented, which placentals require to contain their fetus during gestation periods. A narrow pelvic outlet indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.[39]

The earliest known monotreme was Teinolophos, which lived about 120 million years ago in Australia.[40] Monotremes have some features which may be inherited from the original amniotes such as the same orifice to urinate, defecate and reproduce (cloaca) – as lizards and birds also do –[41] and they lay eggs which are leathery and uncalcified.[42]

Hadrocodium, whose fossils date from approximately 195 million years ago, in the early Jurassic, provides the first clear evidence of a jaw joint formed solely by the squamosal and dentary bones; there is no space in the jaw for the articular, a bone involved in the jaws of all early synapsids.[43]

The earliest clear evidence of hair or fur is in fossils of Castorocauda and Megaconus, from 164 million years ago in the mid-Jurassic. In the 1950s, it was suggested that the foramina (passages) in the maxillae and premaxillae (bones in the front of the upper jaw) of cynodonts were channels which supplied blood vessels and nerves to vibrissae (whiskers) and so were evidence of hair or fur;[44][45] it was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae, as the modern lizard Tupinambis has foramina that are almost identical to those found in the nonmammalian cynodont Thrinaxodon.[24][46] Popular sources, nevertheless, continue to attribute whiskers to Thrinaxodon.[47] Studies on Permiancoprolites suggest that non-mammalian synapsids of the epoch already had fur, setting the evolution of hairs possibly as far back as dicynodonts.[48]

When endothermy first appeared in the evolution of mammals is uncertain, though it is generally agreed to have first evolved in non-mammalian therapsids.[48][49] Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals,[50] but there is evidence that some of their ancestors, perhaps including ancestors of the therians, may have had body temperatures like those of modern therians.[51] Likewise, some modern therians like afrotheres and xenarthrans have secondarily developed lower body temperatures.[52]

The evolution of erect limbs in mammals is incomplete — living and fossil monotremes have sprawling limbs. The parasagittal (nonsprawling) limb posture appeared sometime in the late Jurassic or early Cretaceous; it is found in the eutherian Eomaia and the metatherian Sinodelphys, both dated to 125 million years ago.[53]Epipubic bones, a feature that strongly influenced the reproduction of most mammal clades, are first found in Tritylodontidae, suggesting that it is a synapomorphy between them and mammaliformes. They are omnipresent in non-placental mammaliformes, though Megazostrodon and Erythrotherium appear to have lacked them.[54]

It has been suggested that the original function of lactation (milk production) was to keep eggs moist. Much of the argument is based on monotremes, the egg-laying mammals.[55][56]

Therian mammals took over the medium- to large-sized ecological niches in the Cenozoic, after the Cretaceous–Paleogene extinction event emptied ecological space once filled by non-avian dinosaurs and other groups of reptiles, as well as various other mammal groups.[57] Then mammals diversified very quickly; both birds and mammals show an exponential rise in diversity.[57] For example, the earliest known bat dates from about 50 million years ago, only 16 million years after the extinction of the dinosaurs.[58]

Molecular phylogenetic studies initially suggested that most placental orders diverged about 100 to 85 million years ago and that modern families appeared in the period from the late Eocene through the Miocene.[59] However, no placental fossils have been found from before the end of the Cretaceous.[60] The earliest undisputed fossils of placentals comes from the early Paleocene, after the extinction of the dinosaurs.[60] In particular, scientists have identified an early Paleocene animal named Protungulatum donnae as one of the first placental mammals.[61] however it has been reclassified as a non-placental eutherian.[62] Recalibrations of genetic and morphological diversity rates have suggested a terminal Maastrichtian origin for placentals, and a Paleocene origin for most modern clades.[63]

The earliest known ancestor of primates is Archicebus achilles[64] from around 55 million years ago.[64] This tiny primate weighed 20–30 grams (0.7–1.1 ounce) and could fit within a human palm.[64]

Middle ear - In crown-group mammals, sound is carried from the eardrum by a chain of three bones, the malleus, the incus and the stapes. Ancestrally, the malleus and the incus are derived from the articular and the quadrate bones that constituted the jaw joint of early therapsids.[67]

Tooth replacement - Teeth are replaced once or (as in toothed whales and murid rodents) not at all, rather than being replaced continually throughout life.[68]

Prismatic enamel - The enamel coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from the dentin to the tooth's surface.[69]

The lungs of mammals are spongy and honeycombed. Breathing is mainly achieved with the diaphragm, which divides the thorax from the abdominal cavity, forming a dome convex to the thorax. Contraction of the diaphragm flattens the dome, increasing the volume of the lung cavity. Air enters through the oral and nasal cavities, and travels through the larynx, trachea and bronchi, and expands the alveoli. Relaxing the diaphragm has the opposite effect, decreasing the volume of the lung cavity, causing air to be pushed out of the lungs. During exercise, the abdominal wall contracts, increasing pressure on the diaphragm, which forces air out quicker and more forcefully. The rib cage is able to expand and contract the chest cavity through the action of other respiratory muscles. Consequently, air is sucked into or expelled out of the lungs, always moving down its pressure gradient.[76][77] This type of lung is known as a bellows lung due to its resemblance to blacksmith bellows.[77]

The integumentary system is made up of three layers: the outermost epidermis, the dermis and the hypodermis. The epidermis is typically 10 to 30 cells thick; its main function is to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is 15 to 40 times thicker than the epidermis. The dermis is made up of many components, such as bony structures and blood vessels. The hypodermis is made up of adipose tissue, which stores lipids and provides cushioning and insulation. The thickness of this layer varies widely from species to species;[78]:97marine mammals require a thick hypodermis (blubber) for insulation, and right whales have the thickest blubber at 20 inches (51 cm).[79] Although other animals have features such as whiskers, feathers, setae, or cilia that superficially resemble it, no animals other than mammals have hair. It is a definitive characteristic of the class. Though some mammals have very little, careful examination reveals the characteristic, often in obscure parts of their bodies.[78]:61

Herbivores have developed a diverse range of physical structures to facilitate the consumption of plant material. To break up intact plant tissues, mammals have developed teeth structures that reflect their feeding preferences. For instance, frugivores (animals that feed primarily on fruit) and herbivores that feed on soft foliage have low-crowned teeth specialized for grinding foliage and seeds. Grazing animals that tend to eat hard, silica-rich grasses, have high-crowned teeth, which are capable of grinding tough plant tissues and do not wear down as quickly as low-crowned teeth.[80] Most carnivorous mammals have carnassialiforme teeth (of varying length depending on diet), long canines and similar tooth replacement patterns.[81]

The stomach of Artiodactyls is divided into four sections: the rumen, the reticulum, the omasum and the abomasum (only ruminants have a rumen). After the plant material is consumed, it is mixed with saliva in the rumen and reticulum and separates into solid and liquid material. The solids lump together to form a bolus (or cud), and is regurgitated. When the bolus enters the mouth, the fluid is squeezed out with the tongue and swallowed again. Ingested food passes to the rumen and reticulum where cellulytic microbes (bacteria, protozoa and fungi) produce cellulase, which is needed to break down the cellulose in plants.[82]Perissodactyls, in contrast to the ruminants, store digested food that has left the stomach in an enlarged cecum, where it is fermented by bacteria.[83]Carnivora have a simple stomach adapted to digest primarily meat, as compared to the elaborate digestive systems of herbivorous animals, which are necessary to break down tough, complex plant fibers. The caecum is either absent or short and simple, and the large intestine is not sacculated or much wider than the small intestine.[84]

The mammalian heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers.[85] The heart has four valves, which separate its chambers and ensures blood flows in the correct direction through the heart (preventing backflow). After gas exchange in the pulmonary capillaries (blood vessels in the lungs), oxygen-rich blood returns to the left atrium via one of the four pulmonary veins. Blood flows nearly continuously back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the ventricular relaxation period, the left atrium will contract, pumping blood into the ventricle. The heart also requires nutrients and oxygen found in blood like other muscles, and is supplied via coronary arteries.[86]

Mammalian coats or pelage are colored for a variety of reasons, the major selective pressures including camouflage, sexual selection, communication and physiological processes such as temperature regulation. Camouflage is a powerful influence in a large number of mammals, as it helps to conceal individuals from predators or prey.[87]Aposematism, warning off possible predators, is the most likely explanation of the black-and-white pelage of many mammals which are able to defend themselves, such as in the foul-smelling skunk and the powerful and aggressive honey badger.[88] In arctic and subarctic mammals such as the arctic fox (Alopex lagopus), collared lemming (Dicrostonyx groenlandicus), stoat (Mustela erminea), and snowshoe hare (Lepus americanus), seasonal color change between brown in summer and white in winter is driven largely by camouflage.[89] The green coloration of sloths, however, is caused by algal growths.[90] Differences in female and male coat color may indicate nutrition and hormone levels, important in mate selection.[91] Some primates and marsupials have shades of violet, green, or blue skin on parts of their bodies, indicating some distinct advantage in their largely arboreal habitat due to convergent evolution.[92] Coat color is sometimes sexually dimorphic, as in many primate species.[93]

Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypus and the four species of echidna, lay eggs. The monotremes have a sex determination system different from that of most other mammals.[94] In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.[95]

The mammary glands of mammals are specialized to produce milk, the primary source of nutrition for newborns. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.[96]

Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. Marsupials have a short gestation period, typically shorter than its estrous cycle and gives birth to an undeveloped newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen. This is the plesiomorphic condition among viviparous mammals; the presence of epipubic bones in all non-placental mammals prevents the expansion of the torso needed for full pregnancy.[72] Even non-placental eutherians probably reproduced this way.[97] The placentals are unusual among mammals in giving birth to complete and fully developed young, usually after long gestation periods.[98]

Nearly all mammals are endothermic ("warm-blooded"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in weather and climates too cold for ectothermic ("cold-blooded") reptiles and insects. Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles.[99] Small insectivorous mammals eat prodigious amounts for their size. A rare exception, the naked mole-rat produces little metabolic heat, so it is considered an operational poikilotherm.[100] Birds are also endothermic, so endothermy is not unique to mammals.[101]

Many mammals communicate by vocalizing. Vocal communication serves many purposes, including in mating rituals, as warning calls,[103] to indicate food sources, and for social purposes. Males often call during mating rituals to ward off other males and to attract females, as in the roaring of lions and red deer.[104] The songs of the humpback whale may be signals to females;[105] they have different dialects in different regions of the ocean.[106] Social vocalizations include the territorial calls of gibbons, and the use of frequency in greater spear-nosed bats to distinguish between groups.[107] The vervet monkey gives a distinct alarm call for each of at least four different predators, and the reactions of other monkeys vary according to the call. For example, if an alarm call signals a python, the monkeys climb into the trees, whereas the eagle alarm causes monkeys to seek a hiding place on the ground.[102]Prairie dogs similarly have complex calls that signal the type, size, and speed of an approaching predator.[108] Elephants communicate socially with a variety of sounds including snorting, screaming, trumpeting, roaring and rumbling. Some of the rumbling calls are infrasonic, below the hearing range of humans, and can be heard by other elephants up to 6 miles (9.7 km) away at still times near sunrise and sunset.[109]

To maintain a high constant body temperature is energy expensive – mammals therefore need a nutritious and plentiful diet. While the earliest mammals were probably predators, different species have since adapted to meet their dietary requirements in a variety of ways. Some eat other animals – this is a carnivorous diet (and includes insectivorous diets). Other mammals, called herbivores, eat plants. An herbivorous diet includes subtypes such as granivory (seed eating), folivory (leaf eating), frugivory (fruit eating), nectarivory (nectar eating), gummivory (gum eating) and mycophagy (fungus eating). Some mammals are coprophagous, consuming feces to absorb the nutrients not digested when the food was first ingested.[78]:131–137 An omnivore eats both prey and plants. Carnivorous mammals have a simple digestive tract because the proteins, lipids and minerals found in meat require little in the way of specialized digestion. Plants on the other hand contain complex carbohydrates, such as cellulose. The digestive tract of an herbivore is therefore host to bacteria that ferment these substances, and make them available for digestion. The bacteria are either housed in the multichambered stomach or in a large cecum.[118]

The size of an animal is also a factor in determining diet type (Allen's rule). Since small mammals have a high ratio of heat-losing surface area to heat-generating volume, they tend to have high energy requirements and a high metabolic rate. Mammals that weigh less than about 18 oz (500 g) are mostly insectivorous because they cannot tolerate the slow, complex digestive process of an herbivore. Larger animals, on the other hand, generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (those that prey on larger vertebrates) or a slower digestive process (herbivores).[119] Furthermore, mammals that weigh more than 18 oz (500 g) usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites).[120]

In intelligent mammals, such as primates, the cerebrum is larger relative to the rest of the brain. Intelligence itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. Rats, for example, are considered to be highly intelligent, as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh habitat. In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain smaller than a cat, which must think to outwit its prey.[120]

Tool use by animals may indicate different levels of learning and cognition. The sea otter uses rocks as essential and regular parts of its foraging behaviour (smashing abalone from rocks or breaking open shells), with some populations spending 21% of their time making tools.[121] Other tool use, such as chimpanzees using twigs to "fish" for termites, may be developed by watching others use tools and may even be a true example of animal teaching.[122] Tools may even be used in solving puzzles in which the animal appears to experience a "Eureka moment".[123] Other mammals that do not use tools, such as dogs, can also experience a Eureka moment.[124]

Brain size was previously considered a major indicator of the intelligence of an animal. Since most of the brain is used for maintaining bodily functions, greater ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks. Allometric analysis indicates that mammalian brain size scales at approximately the ⅔ or ¾ exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalisation quotient that can be used as another indication of animal intelligence.[125]Sperm whales have the largest brain mass of any animal on earth, averaging 8,000 cubic centimetres (490 in3) and 7.8 kilograms (17 lb) in mature males.[126]

Self-awareness appears to be a sign of abstract thinking. Self-awareness, although not well-defined, is believed to be a precursor to more advanced processes such as metacognitive reasoning. The traditional method for measuring this is the mirror test, which determines if an animals possesses the ability of self-recognition.[127] Mammals that have 'passed' the mirror test include Asian elephants (some pass, some do not);[128] chimpanzees;[129] bonobos;[130] orangutans;[131] humans, from 18 months (mirror stage);[132] bottlenose dolphins[a][133] killer whales;[134] and false killer whales.[134]

Eusociality is the highest level of social organization. These societies have an overlap of adult generations, the division of reproductive labor and cooperative caring of young. Usually insects, such as bees, ants and termites, have eusocial behavior, but it is demonstrated in two rodent species: the naked mole-rat[135] and the Damaraland mole-rat.[136]

Presociality is when animals exhibit more than just sexual interactions with members of the same species, but fall short of qualifying as eusocial. That is, presocial animals can display communal living, cooperative care of young, or primitive division of reproductive labor, but they do not display all of the three essential traits of eusocial animals. Humans and some species of Callitrichidae (marmosets and tamarins) are unique among primates in their degree of cooperative care of young.[137]Harry Harlow set up an experiment with rhesus monkeys, presocial primates, in 1958; the results from this study showed that social encounters are necessary in order for the young monkeys to develop both mentally and sexually.[138]

A fission-fusion society is a society that changes frequently in its size and composition, making up a permanent social group called the "parent group". Permanent social networks consist of all individual members of a community and often varies to track changes in their environment. In a fission–fusion society, the main parent group can fracture (fission) into smaller stable subgroups or individuals to adapt to environmental or social circumstances. For example, a number of males may break off from the main group in order to hunt or forage for food during the day, but at night they may return to join (fusion) the primary group to share food and partake in other activities. Many mammals exhibit this, such as primates (for example orangutans and spider monkeys),[139] elephants,[140]spotted hyenas,[141] lions,[142] and dolphins.[143]

Solitary animals defend a territory and avoid social interactions with the members of its species, except during breeding season. This is to avoid resource competition, as two individuals of the same species would occupy the same niche, and to prevent depletion of food.[144] A solitary animal, while foraging, can also be less conspicuous to predators or prey.[145]

In a hierarchy, individuals are either dominant or submissive. A despotic hierarchy is where one individual is dominant while the others are submissive, as in wolves and lemurs,[146] and a pecking order is a linear ranking of individuals where there is a top individual and a bottom individual. Pecking orders may also be ranked by sex, where the lowest individual of a sex has a higher ranking than the top individual of the other sex, as in hyenas.[147] Dominant individuals, or alphas, have a high chance of reproductive success, especially in harems where one or a few males (resident males) have exclusive breeding rights to females in a group.[148] Non-resident males can also be accepted in harems, but some species, such as the common vampire bat (Desmodus rotundus), may be more strict.[149]

When two animals mate, they both share an interest in the success of the offspring, though often to different extremes, unless the male and female are perfectly monogamous, meaning that they mate for life and take no other partners (even after the original mate’s death), as with wolves, Eurasian beavers, and otters. The amount of parental care will vary.[150][151] There are three types of polygamy: either one or multiple dominant males have with breeding rights (polygyny), multiple males that females mate with (polyandry), or multiple males have exclusive relations with multiple females (polygynandry). It is much more common for polygynous mating to happen, which, excluding leks, are estimated to occur in up to 90% of mammals.[152] Lek mating occurs in harems, wherein one or a few males protect their harem of females from other males who would otherwise mate with the females, as in elephant seals;[153] or males congregate around females and try to attract them with various courtship displays and vocalizations, as in harbor seals.[154]

Most vertebrates—the amphibians, the reptiles and some mammals such as humans and bears—are plantigrade, walking on the whole of the underside of the foot. Many mammals, such as cats and dogs are digitigrade, walking on their toes, the greater stride length allowing more speed. Digitigrade mammals are also often adept at quiet movement.[155] Some animals such as horses are unguligrade, walking on the tips of their toes. This even further increases their stride length and thus their speed.[156] A few mammals, namely the great apes, are also known to walk on their knuckles, at least for their front legs. Giant anteaters[157] and platypuses[158] are also knuckle-walkers.

Animals will use different gaits for different speeds, terrain and situations. For example, horses show four natural gaits, the slowest horse gait is the walk, then there are three faster gaits which, from slowest to fastest, are the trot, the canter and the gallop. Animals may also have unusual gaits that are used occasionally, such as for moving sideways or backwards. For example, the main human gaits are bipedal walking and running, but they employ many other gaits occasionally, including a four-legged crawl in tight spaces.[159] Mammals show a vast range of gaits, the order that they place and lift their appendages in locomotion. Gaits can be grouped into categories according to their patterns of support sequence. For quadrupeds, there are three main categories: walking gaits, running gaits and leaping gaits.[160]Walking is the most common gait, where some feet are on the ground at any given time, and found in almost all legged animals. Running is considered to occur when at some points in the stride all feet are off the ground in a moment of suspension.[159]

Gibbons are very good brachiators because their elongated limbs enable them to easily swing and grasp on to branches.

Arboreal animals frequently have elongated limbs that help them cross gaps, reach fruit or other resources, test the firmness of support ahead and, in some cases, to brachiate.[161] Many arboreal species, such as tree porcupines, silky anteaters, spider monkeys and possums, use prehensile tails to grasp branches. In the spider monkey, the tip of the tail has either a bare patch or adhesive pad, which provides increased friction. Claws can be used to interact with rough substrates and re-orient the direction of forces the animal applies. This is what allows squirrels to climb tree trunks that are so large to be essentially flat from the perspective of such a small animal. However, claws can interfere with an animal's ability to grasp very small branches, as they may wrap too far around and prick the animal's own paw. Frictional gripping is used by primates, relying upon hairless fingertips. Squeezing the branch between the fingertips generates frictional force that holds the animal's hand to the branch. However, this type of grip depends upon the angle of the frictional force, thus upon the diameter of the branch, with larger branches resulting in reduced gripping ability. To control descent, especially down large diameter branches, some arboreal animals such as squirrels have evolved highly mobile ankle joints that permit rotating the foot into a 'reversed' posture. This allows the claws to hook into the rough surface of the bark, opposing the force of gravity. Small size provides many advantages to arboreal species: such as increasing the relative size of branches to the animal, lower center of mass, increased stability, lower mass (allowing movement on smaller branches) and the ability to move through more cluttered habitat.[161] Size relating to weight affects gliding animals such as the sugar glider.[162] Some species of primate, bat and all species of sloth achieve passive stability by hanging beneath the branch. Both pitching and tipping become irrelevant, as the only method of failure would be losing their grip.[161]

Bats are the only mammals that can truly fly. They fly through the air at a constant speed by moving their wings up and down (usually with some fore-aft movement as well). Because the animal is in motion, there is some airflow relative to its body which, combined with the velocity of the wings, generates a faster airflow moving over the wing. This generates a lift force vector pointing forwards and upwards, and a drag force vector pointing rearwards and upwards. The upwards components of these counteract gravity, keeping the body in the air, while the forward component provides thrust to counteract both the drag from the wing and from the body as a whole.[163]

The wings of bats are much thinner and consist of more bones than that of birds, allowing bats to maneuver more accurately and fly with more lift and less drag.[164][165] By folding the wings inwards towards their body on the upstroke, they use 35% less energy during flight than birds.[166] The membranes are delicate, ripping easily; however, the tissue of the bat's membrane is able to regrow, such that small tears can heal quickly.[167] The surface of their wings is equipped with touch-sensitive receptors on small bumps called Merkel cells, also found on human fingertips. These sensitive areas are different in bats, as each bump has a tiny hair in the center, making it even more sensitive and allowing the bat to detect and collect information about the air flowing over its wings, and to fly more efficiently by changing the shape of its wings in response.[168]

Fossorial creatures live in subterranean environments. Many fossorial mammals were classified under the, now obsolete, order Insectivora, such as shrews, hedgehogs and moles. Fossorial mammals have a fusiform body, thickest at the shoulders and tapering off at the tail and nose. Unable to see in the dark burrows, most have degenerated eyes, but degeneration varies between species; pocket gophers, for example, are only semi-fossorial and have very small yet functional eyes, in the fully fossorial marsupial mole the eyes are degenerated and useless, talpa moles have vestigial eyes and the cape golden mole has a layer of skin covering the eyes. External ears flaps are also very small or absent. Truly-fossorial mammals have short, stout legs as strength is more important than speed to a burrowing mammal, but semi-fossorial mammals have cursorial legs. The front paws are broad and have strong claws to help in loosening dirt while excavating burrows, and the back paws have webbing, as well as claws, which aids in throwing loosened dirt backwards. Most have large incisors to prevent dirt from flying into their mouth.[169]

Fully aquatic mammals, the cetaceans and sirenians, have lost their legs and have a tail fin to propel themselves through the water. Flipper movement is continuous. Whales swim by moving their tail fin and lower body up and down, propelling themselves through vertical movement, while their flippers are mainly used for steering. Their skeletal anatomy allows them to be fast swimmers. Most species have a dorsal fin to prevent themselves from turning upside-down in the water.[170][171] The flukes of sirenians are raised up and down in long strokes to move the animal forward, and can be twisted to turn. The forelimbs are paddle-like flippers which aid in turning and slowing.[172]

Semi-aquatic mammals, like pinnipeds, have two pairs of flippers on the front and back, the fore-flippers and hind-flippers. The elbows and ankles are enclosed within the body.[173][174] Pinnipeds have several adaptions for reducing drag. In addition to their streamlined bodies, they have smooth networks of muscle bundles in their skin that may increase laminar flow and make it easier for them to slip through water. They also lack arrector pili, so their fur can be streamlined as they swim.[175] They rely on their fore-flippers for locomotion in a wing-like manner similar to penguins and sea turtles.[176] Fore-flipper movement is not continuous, and the animal glides between each stroke.[174] Compared to terrestrial carnivorans, the fore-limbs are reduced in length, which gives the locomotor muscles at the shoulder and elbow joints greater mechanical advantage;[173] the hind-flippers serve as stabilizers.[175]Other semi-aquatic mammals include beavers, hippopotamuses, otters and platypuses.[177] Hippos are very large semi-aquatic mammals, and their barrel-shaped bodies have graviportal skeletal structures,[178] adapted to carrying their enormous weight, and their specific gravity allows them to sink and move along the bottom of a river.[179]

Hybrids are offspring resulting from the breeding of two genetically distinct individuals, which usually will result in a high degree of heterozygosity, though hybrid and heterozygous are not synonymous. The deliberate or accidental hybridizing of two or more species of closely related animals through captive breeding is a human activity which has been in existence for millennia and has grown for economic purposes.[215] Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids. Hybrids between different species within the same genus (such as between lions and tigers) are known as interspecific hybrids or crosses. Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids.[216] Natural hybrids will occur in hybrid zones, where two populations of species within the same genera or species living in the same or adjacent areas will interbreed with each other. Some hybrids have been recognized as species, such as the red wolf (though this is controversial).[217]

^Decreased latency to approach the mirror, repetitious head circling and close viewing of the marked areas were considered signs of self-recognition since they don't have arms and can't touch the marked areas.